Thienopyrazine電子施體-受體共軛高分子之合成、性質及其光電元件應用

Semiconducting polymers have been paid great attention because their intensive application of light-emitting diodes, thin film transistors, and photovoltaic devices. The conjugated polymer is a class of semiconducting polymers under intensive research, because it owns tunable optoelectronic properties and good processibility, leading to great potential of device application. The main objective of this thesis is to synthesize a series of conjugated polymers composed of different electron donors and acceptor (mainly quinoxaline) for the fabrication and measurement of field effect transistors and photovoltaic cells. In the first part of this thesis (Chapter 2), five side-chain modified quinoxaline-containing conjugated copolymers with donor-acceptor or donor-acceptor -donor structures were synthesized and characterized, including poly[2,5-didecyloxy- 1,4-phenylene-alt-2,3–bis(4-(2-ethylhexyloxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (POC10DTQ(EHP)),poly[2,5-didecyloxy-1,4-phenylene-alt-2,3-bis(4-(2-ethylhexyl oxy)phenyl)-5,8–dithien-2–yl-quinoxaline] (PFODTQ(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PThQx(EHP)), poly[thiophene-2,5-diyl-alt-2,3-bis(4-(2-ethyl hexyloxy)phenyl)-5,8-dithien-2-yl-quinoxaline] (PThDTQ(EHP)), and poly[2,3bis(4-(2-ethylhexyloxy)phenyl)-5,8-dithien-2-yl- quinoxaline] (PDTQ(EHP)). The hybridization of the high-lying HOMO energy level of the donor and low-lying LUMO energy level of the acceptor could produce a relatively small band gap polymer. The synthesized polymers (except PThDTQ(EHP)) are soluble in common organic solvents. Their band gaps are in the range of 1.57 ~ 1.92 eV and 1.93 ~ 2.14 eV measured by optical spectra and cyclic voltammetry, respectively. PThQx(EHP) has the smallest band gap because it is composed of the donor (thiophene) with higher electron-donating ability with the balanced percentage of the acceptor among the analogous polymers. These copolymers were fabricated into thin film transistors by their chlorobenzene solutions and measured their hole mobility in the range of 4.34×10-3 ~ 4.70×10-5 cm2/(Vs) with on/off ratios in the order of 103 ~104. The smooth and homogeneous surface of PDTQ(EHP) film may be a main factor for the highest charge carrier mobility. Furthermore, the solar cell power conversion efficiencies based on polymers PFODTQ(EHP), POC10DTQ(EHP) and PThQx(EHP) achieved 1.76%, 0.64% and 0.92%, respectively. In the second part of this thesis (Chapter 3), five different carbazole-based copolymers were synthesized by palladium(0)-catalyzed Suzuki coupling reaction, including poly[3,6-(2-ethylhexyl)carbazole-alt-2,1,3-benzothiadiazole] (PCzBTD), poly [3,6-(2-ethylhexyl)carbazole-alt-2,3-dimethyl -5,8-quinoxaline] (PCzDQ), poly[3,6-(2-ethylhexyl)carbazole -alt-2,3-dimethyl-5,8–dithien-2–yl-quinoxaline] (PCzDTQ), poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-quinoxaline] (PCzQx(EHP)) and poly[3,6-(2-ethylhexyl)carbazole-alt-2,3-bis (4-(2-ethylhexyloxy) phenyl)-5,8-dithien-2-yl-quinoxaline] (PCzDTQ(EHP)). The polymers containing quinoxaline with long alkoxy-phenylene side chains have better solubility, resulting larger molecular weights and meanwhile soluble in common organic solvents. Their optical and electrochemical band gaps were measured in the range of 1.91 ~ 2.37 eV and 1.63 ~ 2.34 eV, and the effective intramolecular charge transfer should account for the smallest band gap of PCzDTQ(EHP). Furthermore, by the red shifts of UV-visible spectra, we found that the alkoxy-phenylene side groups of quinoxaline both improve the solubility and enhance the effective conjugation length. In sum, the present study suggests the importance of donor/acceptor strength and backbone structure on the electronic and optoelectronic properties of conjugated polymers, and also demonstrated the potential on optoelectronic device application of these polymers.

Polyfluorenes (PFs) have been widely studied recently due to their potential applications in light-emitting diodes, thin-film transistors, and photovoltaic cells. Manipulation of their electronic structures and morphology of polyfluorenes play key roles for the above applications. In this thesis, both theoretical and experimental investigations were conducted on various fluorene-acceptor copolymers to investigate the effects of acceptor strength on their electronic and optoelectronic properties. Variation on the surface structures and photophysical prperties of fluorene-based rod-coil block copolymer brushes with selective solvents were investigated theoretically and experimentally. In the first part of this thesis (Chapter 2), the theoretical geometries and electronic properties of fluorene-based alternating donor-acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e. HOMO, LUMO level, and band gap. It was found that the geometries of fluorene-based donor-acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene-acceptor conjugated copolymers could be tuned by the geometries or acceptor strength. In the second part of this thesis, fluorene-based donor-acceptor copolymers was synthesized and utilized as the emissive layer of light-emitting diodes. In Chapter 3, a series of novel light-emitting copolymers consisted of 9,9-dihexylfluorene (F) and different acceptor segments, including quinoxaline (Q), 2,1,3-benzothiadiazole (BT) and thieno[3,4-b]-pyrazine (TP), were synthesized by the palladium-catalyzed Suzuki coupling reaction. Three fluorene-acceptor alternating copolymers (PFQ, PFBT, PFTP), six F-TP (PFTP0.5~PFTP35) random copolymers, four random copolymers with three emitting units (PFQTPs and PFBTTPs) were investigated and compared with the parent polyfluorene (PF). The experimental results suggest that the acceptor strength or content significantly affect the electronic and optoelectronic properties. The luminescence characteristics based on the prepared polymers depend on the Forster energy transfer or the intramolecular charge transfer, or heavy-atom fluorescence quenching. The tuning of the electronic and optoelectronic properties could be achieved by incorporating various acceptors or content into the polyfluorenes. The emissive color of these LEDs covers the entire visible region, including red, green, blue, and white. In Chapter 4, the photoluminescence and electroluminescence characteristics of a series of polymer blends based on fluorene-acceptor alternating copolymers were investigated. The dependences of energy transfer and quantum efficiency on the chemical structure of component and on the composition of polymer blend were rationalized. Precise control of the composition of polymer blends resulted in incomplete Forster energy transfer from donor to acceptors. Emission from all the components simultaneously resulted in efficient white emission. In the third part of this thesis, a theoretical model based on dissipative particle dynamics was established to simulate the surface structure of rod-coil block copolymer brushes. The effects of different solvent polarities, block ratios, and grafting densities of brushes on the surface structures were investigated. For the justification of our theoretical model, a novel multifunctional amphiphilic rod-coil block copolymer of the type of PF-b-PPEGMA-b-PPOPS was synthesized by ATRP and the functional rod-coil polymer brush was successfully prepared using reactivity of the silanol-carrying PPOPS block. The comparison between simulated surface structures and AFM images shows highly similar surface structures indicating the feasibility of this theoretical model.

The discovery of oxide electronics is of increasing importance today as one of the most promising new technologies and manufacturing processes for a variety of electronic and optoelectronic applications such as next-generation displays, batteries, solar cells, memory devices, and photodetectors[1]. The high potential use seen in oxide electronics is due primarily to their high carrier mobilities and their ability to be fabricated at low temperatures[2]. However, since the majority of oxide semiconductors are n-type oxides, current applications are limited to unipolar devices, eventually developing oxide-based bipolar devices such as p-n diodes and complementary metal-oxide semiconductors.We have contributed to a wide range of oxide semiconductors and their electronics and optoelectronic device applications. Particularly, we have demonstrated n-type oxide-based thin film transistors (TFT), integrating In2O3-based n-type oxide semiconductors from binary cation materials to ternary cation species including InZnO, InGaZnO (IGZO), and InAlZnO. We have suggested channel/metallization contact strategies to achieve stable and high TFT performance[3, 4], identified vacancy-based native defect doping mechanisms[5], suggested interfacial buffer layers to promote charge injection capability[6], and established the role of third cation species on the carrier generation and carrier transport[7]. More recently, we have reported facile manufacturing of p-type SnOx through reactive magnetron sputtering from a Sn metal target[8]. The fabricated p-SnOx was found to be devoid of metallic phase of Sn from x-ray photoelectron spectroscopy and demonstrated stable performance in a fully oxide-based p-n heterojunction together with n-InGaZnO. The oxide-based p-n junctions exhibited a high rectification ratio greater than 103 at ±3 V, a low saturation current of ~2x10-10, and a small turn-on voltage of -0.5 V.In this presentation, we review recent achievements and still remaining issues in transition metal oxide semiconductors and their device applications, in particular, bipolar applications including p-n heterostructures and complementary metal-oxide-semiconductor devices as well as single polarity devices such as TFTs and memristors. In addition, the fundamental mechanisms of carrier transport behaviors and doping mechanisms that govern the performance of these oxide-based devices will also be discussed.ACKNOWLEDGMENTThis work was supported by the U.S. National Science Foundation (NSF) Award No. ECCS-1931088. S.L. and H.W.S. acknowledge the support from the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS. K.N. was supported by Basic Science Research Program (NRF-2021R11A1A01051246) through the NRF Korea funded by the Ministry of Education.REFERENCES[1] K. Nomura et al., Nature, vol. 432, no. 7016, pp. 488-492, Nov 25 2004.[2] D. C. Paine et al., Thin Solid Films, vol. 516, no. 17, pp. 5894-5898, Jul 1 2008.[3] S. Lee et al., Journal of Applied Physics, vol. 109, no. 6, p. 063702, Mar 15 2011, Art. no. 063702.[4] S. Lee et al., Applied Physics Letters, vol. 104, no. 25, p. 252103, 2014.[5] S. Lee et al., Applied Physics Letters, vol. 102, no. 5, p. 052101, Feb 4 2013, Art. no. 052101.[6] M. Liu et al., ACS Applied Electronic Materials, vol. 3, no. 6, pp. 2703-2711, 2021/06/22 2021.[7] A. Reed et al., Journal of Materials Chemistry C, 10.1039/D0TC02655G vol. 8, no. 39, pp. 13798-13810, 2020.[8] D. H. Lee et al., ACS Applied Materials & Interfaces, vol. 13, no. 46, pp. 55676-55686, 2021/11/24 2021.

In recent years, oxide electronics has emerged as one of the most promising new technologies for a variety of electrical and optoelectronic applications, including next-generation displays, solar cells, batteries, and photodetectors. Oxide electronics have a lot of potential because of their high carrier mobilities and ability to be manufactured at low temperatures. However, the preponderance of oxide semiconductors is n-type oxides, limiting present applications to unipolar devices and stifling the development of oxide-based bipolar devices like p-n diodes and complementary metal-oxide—semiconductors. We have contributed to oxide electronics, particularly on transition metal oxide semiconductors of which the cations include In, Zn, Sn and Ga. We have integrated these oxide semiconductors into thin film transistors (TFTs) as active channel layer in light of the unique combination of electronic and optical properties such as high carrier mobility (5-10 cm“/Vs), optical transparency in the visible regime (>~90%) and mild thermal budget processing (200-400°C). In this study, we achieved four different results. The first result is that unlike several previous reports on oxide p-n junctions fabricated exploiting a thin film epitaxial growth technique (known as molecular beam epitaxy, MBE) or a high-powered laser beam process (known as pulsed laser deposition, PLD) that requires ultra-high vacuum conditions, a large amount of power, and is limited for large-area processing, we demonstrate oxide-based heterojunction p-n diodes that consist of sputter-synthesized p-SnOx and n-IGZO of which the manufacturing routes are in-line with current manufacturing requirements. The second result is that the synthesized p-SnOx films are devoid of metallic Sn phases (i.e., sn? state) with carrier density tuneability and high carrier mobility (> 2 cm2/Vs). The third result is that the charge blocking performance of the metallurgical junction is significantly enhanced by the engineering of trap/defect density of n-IGZO, which is identified using photoelectron microscopy and valence band measurements. The last result is that the resulting oxide-based p-n heterojunction exhibits a high rectification ratio greater than 10° at +3 V (highest among the sputter-processed oxide junctions), a low saturation current of ~2x10 © A, and a small turnon voltage of ~0.5 V. The outcomes of the current study are expected to contribute to the development of p-type oxides and their industrial device applications such as p-n diodes of which the manufacturing routes are in-line with the current processing requirements. Reference: 1. Lee, S.; Paine, D. C., Metallization selection and the performance of amorphous In-Zn-O thin film transistors. Applied Physics Letters 2014, 104 (25), 252103. 2. Liu, M.; Kim, H.; Wang, X.; Song, H. W.; No, K.; Lee, S., Carrier Density-Tunable Work Function Buffer at the Channel/Metallization Interface for Amorphous Oxide Thin-Film Transistors. ACS Applied Electronic Materials 2021, 3 (6), 2703-2711. 3. Chu, X., Zhuang, S., Chi, C., Xu, H., Du, G., Dong, X., & Yin, J. (2017). Study on the electroluminescence properties of the P-nimgo:Li/mgo/n-zno nanowires/ito heterojunction. Journal of Luminescence, 187, 486-491. 4. Kawazoe, H.; Yanagi, H.; Ueda, K.; Hosono, H., Transparent p-Type Conducting Oxides: Design and Fabrication of p-n Heterojunctions. MRS Bull. 2000, 25 (08), 28-36. 5. Mo, X., Fang, G., Long, H., Li, S., Wang, H., Chen, Z., Huang, H., Zeng, W., Zhang, Y., & Pan, C. (2014). Unusual electroluminescence from n-ZnO@i-MgO core-shell nanowire color-tunable light-emitting diode at reverse bias. Phys. Chem. Chem. Phys., 16(20), 9302-9308. 6. Nomura, K.; Ohta, H.; Takagi, A.; Kamiya, T.; Hirano, M.; Hosono, H., Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 2004, 432 (7016), 488-492.

Polymer photovoltaic cells currently achieve power conversion efficiencies (PCE) above 10% on lab scale. To compete with the efficiencies above 20% of inorganic solar cells, understanding and elimination of all the loss channels is necessary. This thesis investigates charge generation and recombination processes in polymer-fullerene solar cells with the aim of elucidating the nature of such losses. Chapter 1 provides a general introduction to the subject of the thesis and an overview of the most relevant literature. In Chapter 2 the effect of the thermodynamic driving force on the efficiency of photoinduced charge transfer between semiconducting polymers and fullerene is investigated. A polyfluorene copolymer is mixed with a variety of fullerene mono- and bisadducts having different LUMO energy levels. The difference in LUMO energy of the acceptors results in different energies of the charge transfer (CT) states in the blends. By spectroscopic characterization of bulk heterojunction thin films and photovoltaic devices it is found that the CT state needs to be at least 0.1 eV lower in energy than the singlet excited state of the fullerene to be formed efficiently. The central finding of Chapters 3 to 5 is that the nanoscale morphology of the polymer-fullerene heterojunction strongly influences the CT state dissociation and recombination. Blends composed of a small optical bandgap semiconducting polymer (PCPDTBT) mixed with fullerene are studied with various techniques. The nanoscale phase separation of these blends is controlled by adding high-boiling point co-solvents to the solutions used to process the thin films. Without co-solvent the blends are very finely mixed and the addition of co-solvents increases the phase separation. In blends that are too finely mixed we observe recombination of the CT state to the triplet state of the polymer (Chapter 3). We monitor the formation of triplets and charges in the thin films by means of photo-induced absorption (PIA).The recombination to the triplet state is a loss mechanism, in competition with the dissociation of the CT state into free charges. This process is reduced in favor of free charge formation when the phase separation is increased, correlating with the increased device performance. Specifically, increased fill factor and short circuit current are observed in the devices with optimized morphology. These results show that photophysical processes are controlled by the nanoscale morphology. In Chapter 4 we perform charge extraction experiments on solar cells having different nanoscale phase separation and we support the results with PIA experiments on corresponding thin films. In the charge extraction and PIA experiments we study the same set of long lived charges. We confirm that CT recombination (to the triplet and to the ground state) is largely responsible for the poor device performances of not-optimized blends. Finally, in Chapter 5 we show a correlation between population of triplet excited states and decreased photo-stability of the blends. We suggest that triplet states can lead to the formation of singlet oxygen, which in turn can initiate destructive chemical reactions in the polymer chains. In Chapters 6 and 7 the influence of excess photon energy (i.e. the difference between the energy of the absorbed photon and that of the CT state) in the CT state dissociation is investigated by means of charge extraction and spectral response measurements in different polymer:fullerene blends. We find that excess energy is not necessary for CT dissociation in blends with coarse phase separation. A measurable influence of the excess photon energy on CT dissociation is observed only in PCPDTBT:PCBM finely dispersed blends, which were shown in the previous chapters to be strongly affected by CT state recombination. Chapter 8 investigates what is limiting the photo-current of solar cells close to the open-circuit voltage. It is found that under these conditions space-charge can build up, so that the photocurrent becomes space-charge limited. This condition is reached only in blends where CT dissociation is efficient at low electric fields. Impedance spectroscopy is the technique used here. The experimental results are supported by an analytical model and by drift diffusion simulations. Summarizing, in this thesis it is shown that the CT state is a critical intermediate state in the charge generation process in polymer solar cells. The energy of the CT state needs to be low enough to convert neutral excitations into charges. The blend morphology is a crucial factor determining the branching ratio between dissociation into free carriers and recombination to the ground state or to the triplet state of the polymer.

The self-assembly of block copolymers into nanostructure with novel morphology and property has attracted an increasing interest as a new approach for nanotechnology. In particular, block copolymers containing conducting polymer segments such as poly (phenylene vinylene) (PPV) are technologically important since PPVs are promising material for applications in low cost, large area and flexible organic optoelectronics. In this thesis, I described a novel method to synthesize poly(2,5-di(2’-ethylhexyloxy)-1-4-phenylenevinylene)-b-poly(ethylene oxide) (DEH-PPV-b-PEO) block copolymers via a coupling reaction of end-functionalized polymer building blocks such as 1,3-dipolar cycloaddition reaction between an alkyne terminated DEH-PPV and an azide terminated PEO. Aldehyde terminated monodispersed DEH-PPV homopolymers were synthesized using Siegrist polycondensation of 2,5-di(2”-ethylhexyloxy)-4’-methyl-N-benzylideneaniline and subsequently substitution of the terminal imino group with acid. Alkyne terminated PPV was obtained by reaction of aldehyde terminated PPV with propargyl bromide. Azide terminated PEO were synthesized by tosylation of the hydroxyl functionality of poly(ethylene glycol) methyl ether and subsequent substitution with sodium azide. The synthesized PPV-b-PEO block copolymers were characterized by various techniques including GPC, NMR, DSC, TGA, solution UV-Vis and PL. The microstructure and domain spacing for the synthesized block copolymers were further examined using small-angle x-ray scattering technique (SAXS). These block copolymers microphase separate and self-assemble into nanostructures of lamellae irrespective to the different PEO fractions, indicating the rod nature of PPV block. PPV-b-PEO shows significant solvatochromic transformation in different solvent/non-solvent mixture. The optical property of the PPV-b-PEO solution can be explained on the basis of aggregation, agglomeration, and collapse of isolated chains of PPV as well as the micelle and inverse micelle structures of PPV-b-PEO. The synthesis of PPV-b-PEO opens up the opportunity to study amphiphilic rod-coil block copolymers for future application.

This study presents the design and analysis of five acceptor–donor–acceptor (A–D–A) type donor molecules ( QABT1 – QABT5 ) by incorporating 2,1,3‐benzothiadiazole (BT) derivatives as electron‐withdrawing acceptor groups into a synthesized reference molecule ( R ) for organic solar cell (OSC) applications. Density functional theory (DFT) and time‐dependent DFT (TD‐DFT) methods were employed to explore the impact of acceptor modifications on the structural, electronic, optical, charge transport, and photovoltaic properties of the designed molecules. The results indicate that introducing strong electron‐withdrawing groups significantly enhances overall device performance. The structural modifications lead to reduced HOMO–LUMO band gaps (2.02–2.36 eV), facilitating efficient charge transfer (CT) and extending the absorption spectra. All designed molecules exhibit broader and red‐shifted absorption in both gaseous and solvent phases, along with lower optical band gaps and reduced exciton binding energies, improving exciton dissociation and charge transport efficiency compared to the reference molecule. Among the designed molecules, QABT4 and QABT5 emerge as the most promising candidates, exhibiting a small optical band gap (1.75 eV), extended excited‐state lifetime (9.65 ns), and low reorganization energies for electrons ( λ e ∼97 meV) and holes ( λ ℎ ∼120 meV). Additionally, the strong push‐pull mechanism results in efficient intramolecular CT, with above 85% CT excitations. These properties contribute to an enhanced short‐circuit current density (J sc ∼13.73 mA/cm 2 ), high open‐circuit voltage (V oc ∼1.46 V) with minimal energy loss (0.57 eV), and a theoretical power conversion efficiency (PCE) up to ∼18%. These findings underscore QABT4 and QABT5 as promising candidates for high‐performance OSCs, paving the way for next‐generation optoelectronic applications.

Tin monoxide (SnO) has attracted attention due to its p-type character and capability of ambipolar conductivity when properly doped, properties that are beneficial for the realization of complementary oxide thin film transistors technology, transparent flexible circuits and optoelectronic applications in general. However, its small fundamental band gap (0.7 eV) limits its applications as a solar energy material, therefore tuning its electronic properties is necessary for optimal performance. In this work, we use density functional theory (DFT) calculations to examine the electronic properties of the Sn1−xPbxO ternary oxide system. Alloying with Pb by element substitution increases the band gap of SnO without inducing defect states in the band gap retaining the anti-bonding character of the valence band maximum which is beneficial for p-type conductivity. We also examine the properties of the SnO/PbO heterojunction system in terms of band alignment and the effect of the most common intrinsic defects. A broken gap band alignment for the SnO/PbO heterojunction is calculated, which can be attractive for energy conversion in solar cells, photocatalysis and hydrogen generation.

Zinc nitride (Zn3N2) comprises earth-abundant elements, possesses a small direct bandgap, and is characterized by high electron mobility. While these characteristics make the material a promising compound semiconductor for various optoelectronic applications, including photovoltaics and thin-film transistors, it commonly exhibits unintentional degenerate n-type conductivity. This degenerate character has significantly impeded the development of Zn3N2 for technological applications and is commonly assumed to arise from incorporation of oxygen impurities. However, consistent understanding and control of the role of native and impurity defects on the optoelectronic properties of this otherwise promising semiconductor have not yet emerged. Here, we systematically synthesize epitaxial Zn3N2 thin films with controlled oxygen impurity concentrations of up to 20 at % by plasma-assisted molecular beam epitaxy (PA-MBE). Contrary to expectations, we find that oxygen does not lead to degenerate conductivity but instead serves as a compensating defect, the control of which can be used to achieve nondegenerate semiconducting thin films with free electron concentrations in the range of 1017 cm-3, while retaining high mobilities in excess of 200 cm2 V-1 s-1. This understanding of the beneficial role of oxygen thus provides a route to controllably synthesize nondegenerate O-doped Zn3N2 for optoelectronic applications.

We have investigated the electronic structure and carrier mobility of monolayer black phosphorus nanoribbons (BPNRs) using density functional theory combined with Boltzmann transport method with relaxation time approximation. It is shown that the calculated ultrahigh electron mobility can even reach the order of 10(3) to 10(7) cm(2) V(-1) s(-1) at room temperature. Owing to the electron mobility being higher than the hole mobility, armchair and diagonal BPNRs behave like n-type semiconductors. Comparing with the bare BPNRs, the difference between the hole and electronic mobilities can be enhanced in ribbons with the edges terminated by H atoms. Moreover, because the hole mobility is about two orders of magnitude larger than the electron mobility, zigzag BPNRs with H termination behave like p-type semiconductors. Our results indicate that BPNRs can be considered as a new kind of nanomaterial for applications in optoelectronics, nanoelectronic devices owing to the intrinsic band gap and ultrahigh charge mobility.

The development of high-performance p-type oxides with wide band gap and high hole mobility is critical for the application of oxide semiconductors in back-end-of-line (BEOL) complementary metal-oxide-semiconductor (CMOS) devices. SnO has been intensively studied as a high-mobility p-type oxide due to its low effective hole mass resulting from the hybridized O-2p/Sn-5s orbital character at the valence band edge. However, SnO has a very small band gap (∼0.7 eV) for practical p-type oxide devices. In this work, we report an engineering method to enhance the band gap and hole mobility in SnO. It is found that both the band gap and the hole mobility of a layer-structured SnO increase with the interlayer stacking spacing change. By exploiting this unique electronic structure feature, we propose expanding the interlayer spacing by interlayer intercalation to engineer the band gap and p-type mobility in SnO. Small molecules like NH3 and CH4 are shown to be capable of expanding the interlayer spacing and of increasing the band gap and hole mobility in SnO and thus could potentially serve as the interlayer intercalants. The results provide a viable way for the experimental realization of wide-band-gap and high hole-mobility p-type SnO for BEOL vertical CMOS device applications.

Realizing p-channel semiconducting polymers with good hole mobility, solution processibility, and air stability is an important step forward in the chemical manipulation of charge transport in polymeric solids and in the development of low-cost printed electronics. We report here the synthesis and full characterization of the dithienosilole- and dibenzosilole-based homopolymers, poly(4,4-di-n-hexyldithienosilole) (TS6) and poly(9,9-di-n-octyldibenzosilole) (BS8), and their mono- and bithiophene copolymers, poly(4,4-di-n-hexyldithienosilole-alt-(bi)thiophene) (TS6T1, TS6T2) and poly(9,9-di-n-octyldibenzosilole-alt-(bi)thiophene) (BS8T1, BS8T2), and examine in detail the consequences of introducing dithienosilole and dibenzosilole cores into a thiophene polymer backbone. We demonstrate air-stable thin-film transistors (TFTs) fabricated under ambient conditions having hole mobilities as large as 0.08 cm(2)/V x s, low turn-on voltages, and current on/off ratios > 10(6). Additionally, unencapsulated TFTs fabricated under ambient conditions are air-stable, an important advance over regioregular poly(3-hexylthiophene) (P3HT)-based devices. Density functional theory calculations provide detailed insight into the polymer physicochemical and charge transport characteristics. A direct correlation between the hole injection barrier and both TFT turn-on voltage and TFT polymer hole mobility is identified and discussed, in combination with thin-film morphological characteristics, to explain the observed OTFT performance trends.

Organic semiconductors are an interesting class of materials for application in a wide range of emerging opto-electronics and electrical devices, such as light emitting diodes, thin-film transistors, solar cells, sensors, and thermoelectrics. However, despite the tremendous efforts on the development of the high-performance materials, device architectures, and manufacturing techniques; crucial challenges still persist and will need to be addressed in order to fully exploit the potential of this class of materials. In this respect, the underlying physics of charge transport in organic semiconductors is still not fully understood. Particularly, solution processed organic semiconductors have complex microstructures. In contrast to inorganic semiconductors, carrier transport in these partially ordered films cannot be described using models for either amorphous or crystalline semiconductors due to their uncontrolled morphology and alignment in the charge transport domain. Further, organic semiconductors are intrinsic and must be doped either with chemical or molecular dopants, carrier injection via external contacts, or photoexcitation in order to transport charge. In this regard, current flow may cause unwanted device heating in organic electrical devices due to high resistance of organic layers. Again, the low thermal conductivity of organic semiconductors prevents rapid heat dissipation from the device, which can have negative consequences on device lifetime and performance. Doping in organic semiconductors can significantly impact the molecular backbone, thin film morphology, and corresponding charge transport nature. The challenge is to experimentally untangle these intertwined physical and chemical phenomena. This thesis extensively investigates charge transport in conjugated organic semiconductors, with a focus on macromolecules such as conjugated polymers that are applied in opto-electronic and energy applications. The goal was to elucidate the impact of electrical and molecular doping in different conjugated systems. I also studied the role of different chemical structures on charge transport efficiency. Chapter 1 provides the general background on electronic structure, charge transport, current-voltage characteristics, and vibrational spectroscopy of organic semiconductors. Chapter 2 includes the details of the materials, device structures, and the experimental procedures used for the experiments. Chapter 3 describes how electrical doping impacts the charge transport of organic diodes. Here, I used a novel approach based on in-situ Raman spectroscopy coupled with electrical measurements to unravel the effects of electrical-self heating in the organic semiconductor layer of a working diode. Chapter 4 describes the influence of p-type and n-type doping in molecular structure, thus charge transport of a donor and acceptor-type organic semiconductor, respectively. Especially, this chapter shows how oxidation (p-doping) and reduction (n-doping) of a pristine conjugated chain via doping influences the molecular structure and property by using vibrational spectroscopy, and electrical study. Chapter 5 demonstrates how the structural functionalization of conjugated polymer substrates modifies the charge transport in the working diodes and solar cells. Further, this chapter provides the characterization of novel acceptor polymers for all-polymer solar cell and single-component polymer solar cell applications. Finally, Chapter 6 summarizes the research described in this thesis and outlines the future directions toward an improved understanding of the device physics of emerging organic opto-electronics.

We report the molecular engineering of new small molecule-based medium band gap nonfullerene acceptors containing a strong electron acceptor backbone of DPP attached to the [Formula: see text]-spacer-based furan linker. We systematically designed five new designed molecules (H1−H5) and studied their structural, optical, photovoltaic and optoelectronic characteristics. We employed advanced quantum chemical approaches to characterize these molecules (H1–H5) and synthetic reference molecule R. The designed molecules exhibit improved light absorption and narrower energy bandgaps in contrast to R. Moreover, a lower excitation energy of these molecules (H1−H5) enables easier excitation and phase separation in the excited states. Analysis of the geometric and physiochemical properties provides evidence of the enhanced photovoltaic and optoelectronic characteristics of these molecules in comparison to the R molecule and other molecules with similar structures that have been previously reported. Among all these designed series, H3 stands out with a minimal 1.99 eV band gap, excellent hole ([Formula: see text] 0.0079 eV) and electron ([Formula: see text] 0.0076 eV) mobility, low binding energy ([Formula: see text] 0.42 eV) and highly red-shifted absorption characteristics. Furthermore, these molecules also displayed good electron and hole mobilities, indicating their tremendous potential in producing high-efficiency organic solar cells. Therefore, this study showcases the molecular modeling and effectiveness of these DPP and furan-containing molecules and their promising potential for organic photovoltaics.

Recently, we have reported the synthesis and optical and electronic properties of Sb2S3 nanowires with small band gap. In order to prove that the synthesized nanowires can be considered as a candidate material for solar cells and in order to obtain I–V characteristics, two very simple cells based on synthesized Sb2S3 nanowires/natural dye composite were fabricated. Exponential growth of the I–V curves after illumination revealed that the cells could work as electricity generators. A better current response was observed for the cell made of synthesized powder with smaller band gap. The efficiency of the cells is quite low, but this was an attempt to create a solar cell in order to better understand the properties of the synthesized Sb2S3 semiconductor and the processes that occur in the cell. © 2015 American Institute of Chemical Engineers Environ Prog, 35: 512–516, 2016